The present invention relates to a piezoelectric sound monitor capable of converting acoustic waves to an electric signal.
The xe2x80x9cpiezoelectric effectxe2x80x9d is the appearance of an electric potential and current across certain faces of a crystal when it is subjected to mechanical stresses. Due to their capacity to convert mechanical deformation into an electric voltage, piezoelectric crystals have been broadly used in devices such as transducers, strain gauges and microphones. However, before the crystals can be used in many of these applications they must be rendered into a form which suits the requirements of the application. In many applications, especially those involving the conversion of acoustic waves into a corresponding electric signal, piezoelectric membranes have been used.
Piezoelectric membranes are typically manufactured from polyvinylidene fluoride plastic film. The film is endowed with piezoelectric properties by stretching the plastic while it is placed under a high-poling voltage. By stretching the film, the film is polarized and the molecular structure of the plastic aligned. A thin layer of conductive metal (typically nickel-copper) is deposited on each side of the film to form electrode coatings to which connectors can be attached.
Piezoelectric membranes have a number of attributes that make them interesting for use in sound detection, including:
a wide frequency range of between 0.001 Hz to 1 GHz;
a low acoustical impedance close to water and human tissue;
a high dielectric strength;
a good mechanical strength; and
piezoelectric membranes are moisture resistant and inert to many chemicals.
Due in large part to the above attributes, piezoelectric membranes are particularly suited for the capture of acoustic waves and the conversion thereof into electric signals and, accordingly, have found application in the detection of body sounds. In this regard, sound detecting devices have used piezoelectric membranes as mechano-electric transducers where the piezoelectric membrane becomes temporarily polarised when subjected to a physical force with the direction and the magnitude of the polarisation depending on the magnitude of the force applied.
EPO Patent No. EP 0 716 628 granted to Kassal et al. on Dec. 2, 1998 discloses a disposable acoustic pad sensor including a piezoelectric membrane bonded to a flexible substrate for the detection of heart sounds. The sensor is applied to a patient""s skin with an adhesive or electrode cream and flexes with heartbeat. Similarly, PCT application to Gavrieli et al., published on Oct. 21, 1999 under number WO 99/53277 discloses a device for detecting sounds generated within a patient""s body comprising a piezoelectric sensor placed on the surface of the body and an electronic circuitry for rejecting airborne sounds such as speech. The piezoelectric material in both these sensor and device is typically bonded to a semi-rigid substrate with flexing of the substrate being sensed via the piezoelectric material.
Sensors have also been developed for detecting body sounds where amplification and other signal processing electronics are located within the sensor and/or proximate to the sensor. PCT application (Smith) published on May 17th, 2001 under number WO 01/34033 discloses a stethoscope transducer comprising a diaphragm and co-located signal amplification and filtering electronics. Similarly, PCT application (Sullivan et al.) published on Dec. 27th, 2001 under number WO 01/97691 discloses a biophysical sensor comprised of a piezoelectric membrane and signal amplification and filtering electronics encapsulated in a single package.
The present invention relates to a piezoelectric sound monitor comprising piezoelectric membrane including an inner face, an outer face, and first and second peripheral portion. The piezoelectric sound monitor also comprises a piezoelectric membrane support structure including an outer face, two first mutually facing membrane-clamping walls, a first electrically conductive area, two second mutually facing membrane-clamping walls, and a second electrically conductive area. The inner face of the piezoelectric membrane is applied to the outer face of the support structure. The first peripheral portion of the piezoelectric membrane is clamped between the two first walls, and the first electrically conductive area is located on one of the two first walls for electrically contacting the inner face of the piezoelectric membrane. The second peripheral portion of the piezoelectric membrane is clamped between the two second walls, and the second electrically conductive area is located on one of the two second walls for electrically contacting the outer face of the piezoelectric membrane.
The present invention also relates to a piezoelectric sound monitor, comprising:
a frame defining a window and having first and second faces;
a piezoelectric membrane extending across the window of the frame and including an outer face, an inner face applied to the first face of the frame, and first and second generally opposite peripheral portions;
a board having a first face toward the second face of the frame, a second face opposite to the first face, a first electrically conductive area on the first face of the board, and a second electrically conductive area on the second face of the board; and
a housing connected to the frame and having an inner face toward the second face of the board.
The first peripheral portion of the piezoelectric membrane is bent over the second face of the frame and clamped between the second face of the frame and the first electrically conductive area whereby the first electrically conductive area is in electrical contact with the outer face of the piezoelectric membrane.
In the same manner, the second peripheral portion of the piezoelectric membrane is bent over the second face of the board and is clamped between the second electrically conductive area and the inner face of the housing whereby the second electrically conductive area is in electrical contact with the inner face of the piezoelectric membrane.
The present invention is further concerned with a method for fabricating a sound monitor for the detection of sounds, comprising:
clamping a first peripheral portion of a piezoelectric membrane to a first contact area such that a first face of the piezoelectric membrane is in contact with the first contact area;
spreading the piezoelectric membrane across a window of a frame; and
clamping a second peripheral portion of the piezoelectric membrane to a second contact area such that a second surface of the piezoelectric membrane is in contact with the second contact area.